logstash is a tool for managing events and logs. You can use it to collect
logs, parse them, and store them for later use (like, for searching).
Speaking of searching, logstash comes with a web interface for searching and
drilling into all of your logs.

It is fully free and fully open source. The license is Apache 2.0, meaning
you are pretty much free to use it however you want in whatever way.

Ship logs from any source, parse them, get the right timestamp, index them,
and search them.

All your logs from all over your infrastructure in one place - with
searching and graphing. Since we can easily parse text-based logs, you can
query for more precise things like, all 404 http errors, nagios critical alerts
in hard state, or mail server faults - all without accidentally finding logs
with the word ‘404’ or ‘critical’ in the wrong place.